Simplify FixtureResolver to reuse filtering logic

Previously FixtureResolver had a copy-pasted version of the filtering
already done by OptimizedFileSystemResolver. This PR replaces this by
extracting the two places actual filesystem operations into separate
methods and overriding those.

It would be nice to not rely on overriding methods at all, and to
extract the actual filtering into a separate, reusable class, but I
don't want to do that until some other changes are made to the
filtering.

This should also make FixtureResolver much more accurate to
OptimizedFileSystemResolver, by also creating and caching the
UnboundTemplate classes, which de-duplicate templates.

actionview/lib/action_view/template/resolver.rb

@@ -204,9 +204,13 @@ module ActionView
         end
       end
 
+      def source_for_template(template)
+        Template::Sources::File.new(template)
+      end
+
       def build_unbound_template(template, virtual_path)
         handler, format, variant = extract_handler_and_format_and_variant(template)
-        source = Template::Sources::File.new(template)
+        source = source_for_template(template)
 
         UnboundTemplate.new(
           source,
@@ -316,14 +320,22 @@ module ActionView
     end
 
     private
-      def find_template_paths_from_details(path, details)
+      def find_candidate_template_paths(path)
         # Instead of checking for every possible path, as our other globs would
         # do, scan the directory for files with the right prefix.
         query = "#{escape_entry(File.join(@path, path))}*"
 
+        Dir[query].reject do |filename|
+          File.directory?(filename)
+        end
+      end
+
+      def find_template_paths_from_details(path, details)
+        candidates = find_candidate_template_paths(path)
+
         regex = build_regex(path, details)
 
-        Dir[query].uniq.reject do |filename|
+        candidates.uniq.reject do |filename|
           # This regex match does double duty of finding only files which match
           # details (instead of just matching the prefix) and also filtering for
           # case-insensitive file systems.

actionview/lib/action_view/testing/resolvers.rb

@@ -27,34 +27,17 @@ module ActionView #:nodoc:
     end
 
     private
-      def query(path, exts, _, locals, cache:)
-        regex = build_regex(path, exts)
-
-        @hash.select do |_path, _|
-          ("/" + _path).match?(regex)
-        end.map do |_path, source|
-          handler, format, variant = extract_handler_and_format_and_variant(_path)
-
-          Template.new(source, _path, handler,
-            virtual_path: path.virtual,
-            format: format,
-            variant: variant,
-            locals: locals
-          )
-        end.sort_by do |t|
-          match = ("/" + t.identifier).match(regex)
-          EXTENSIONS.keys.reverse.map do |ext|
-            if ext == :variants && exts[ext] == :any
-              match[ext].nil? ? 0 : 1
-            elsif match[ext].nil?
-              exts[ext].length
-            else
-              found = match[ext].to_sym
-              exts[ext].index(found)
-            end
-          end
+      def find_candidate_template_paths(path)
+        @hash.keys.select do |fixture|
+          fixture.start_with?(path.virtual)
+        end.map do |fixture|
+          "/#{fixture}"
         end
       end
+
+      def source_for_template(template)
+        @hash[template[1..template.size]]
+      end
   end
 
   class NullResolver < PathResolver